Expose reading errors

[groue]

This pull request addresses #147 and changes GRDB so that it exposes reading errors.

Until now, GRDB would assume that both SQLite and the database file(s) were fundamentally trustable. Well, the file system is not trustable. Applications have to be able to deal with I/O errors.

The fetching methods have thus been updated:

-Type.fetch(...) -> DatabaseSequence<Type>
-Type.fetchAll(...) -> [Type]
-Type.fetchOne(...) -> Type?
+Type.fetchCursor(...) throws -> DatabaseCursor<Type>
+Type.fetchAll(...) throws -> [Type]
+Type.fetchOne(...) throws -> Type?

Usage:

// Fetch cursors
let rows = try Row.fetchCursor(db, ...) // DatabaseCursor<Row>
while let row = try rows.next() { ... } // Row

// Fetch arrays
let rows = try Row.fetchAll(db, ...)    // [Row]

// Fetch individual values:
let row = try Row.fetchOne(db, ...)     // Row?

[groue]

@kdubb, @swiftlyfalling This PR looks ready.

Errors from FetchedRecordsController and DatabaseStorage are not handled yet, but this is not a regression: we can deal with those errors in future PRs. I encourage you to check out the DatabaseCursor branch and evaluate this new version (DatabaseCursor has been merged into master).

See:

• CHANGELOG
• Updated documentation for Fetching Methods
• Updated documentation for Error Handling

[groue]

All right, folks! GRDB v0.91.0 has shipped :-) Thanks again @kdubb for the kick!

[groue]

@kdubb @swiftlyfalling I'm currently writing an app that needs to properly handle database errors on a locked device. I can not tell you how much GRDB owes to you ❤️

[swiftlyfalling]

@groue: Yes, that's (unfortunately) an all-too-frequent issue if one wants to support background execution & Data Protection on an iOS device.

These changes are fantastic and make it all possible. 🎉